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Glazing gains

The stadium client's decision to chose polymethyl methacrylate roof panels again is not surprising, given the material's continuing popularity in Europe and the US.

Polycarbonate may have taken over most of the UK market for plastic external glazing in recent years, largely because of its greater toughness and impact resistance, but PMMA's superior optical properties are keeping it in pole position elsewhere.

And then there is PMMA's track record.

First produced around 60 years ago and best known at first under the Plexiglas trade name, PMMA achieved public acceptance during the Second World War, when it revolutionised the design of aircraft cockpit canopies. The P51-D Mustang fighter is the classic example of these first generation clear thermoplastic canopies.

Although PMMA's impact resistance is not particularly high, any fragments formed are generally less lethal than those from glass. Light transmission extends well into the ultra violet region of the spectrum, and degenerates only slowly under the effects of weathering.

But weathering, and especially surface abrasion, can render PMMA panels vulnerable to low energy brittle fracture after quite minor impacts.

High ambient temperatures can also pose problems. Most experts recommend that PMMA should not be used where its temperature is likely to exceed 80C.

Its behaviour in fire is another drawback It depolymerises above 170C to form a highly flammable monomer, and it tends to drip flaming gobbets of molten material when it burns.

For this reason transparent polyvinyl chloride is often specified as an alternative to PMMA.

PVC is relatively resistant to ignition and burning, does not depolymerise and, in a fire, produces a black char rather than burning drips.

Its durability is generally much lower than PMMA, even when blended with stabilisers, and it can become brittle after prolonged weathering.

PVC's biggest drawback, however, is that it can be softened and blackened by ambient temperatures as low as 55C to 60C.

By contrast, polycarbonate's only real drawback would appear to be inferior light transmission. Its resistance to impact and high ambient temperatures is much higher than either PMMA or PVC, and its behaviour in fire is markedly better than PMMA.

But, while the effect of weathering on the impact resistance of polycarbonate is minimal, polycarbonate glazing tends to yellow, lose its gloss and suffer from surface erosion after prolonged exposure.

The Building Research Establishment has carried out accelerated weathering tests on a wide range of plastic glazing materials and the results are discussed in the new Digest 430 - Plastics external glazing due to be published next month.

Dave Parker

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